Furnace and heating method



April 4, 1950 D. P. COOPER ET AL 2,502,828

FURNACE AND HEATING METHOD Filed Jan. 23, 1947 2 Sheets-Sheet l III/ll! 11/ as" a:

INVENTOR YDAVID P.COOPER & EAR NEST D. SEVENICH April 4, 1950 D. P. COOPER ET AL FURNACE AND HEATING METHOD 2 Sheets-Sheet 2 Filed Jan. 23, 1947 INVENTOR DAVID P.COOPER & EARNEST D. SEVENICH Patented Apr. 4, 1950 UNITED STATES PATENT OFFICE FURNACE AND HEATING DETHOD David P.

Cooper and Earnest D.

Sevenich,

Youngstown, Ohio, assignors to The Cold Metal Products Company, Youngstown, Ohio, a corporation of Ohio Application January 23, 1947, Serial No. 723,852

type disclosed in copending applications Serial Nos. 497,523 (Patent No. 2,417,063) and 629,085 and a method or heating a charge therein. A portion of the subject matter claimed in the present application is disclosed but not claimed in application Serial No. 629,085.

In one of its aspects the invention has to do with improvements in material heating furnaces which render such furnaces more efficient in operation and simpler and less costly to build and operate than similar furnaces heretofore available. In certain of its aspects the invention is especially adaptable for the annealing of coils of strip so we have shown in the drawings a furnace of the rotating annular hearth type employed in the annealing of coils of strip.

We provide, in a furnace, a traveling support for supporting material to be heated, a, burner carried by the support, a fuel supply separate from the support and connections between the burner and the fuel supply for supplying fuel to the burner at any point throughout a predetermined range of travel of the support. Normally the traveling support supports at intervals therealong masses to be heated and a series of burners is provided, the burners being carried by the support, one burner being arranged to heat each of the masses supported by the support while the support travels. In one form of structure the travelin support is a rotary hearth, as in the type of furnace selected for illustration.

The traveling support or hearth is adapted to support a coil or a series of coils disposed at intervals along the support to be heated and the burner or burners carried by the support is or are desirably disposed in position generally coaxial with the coil or coils and adapted to heat the same internally.

Means are preferably provided enclosing each coil but having an opening therethrough generally along the axis of the coil. Such means may take the form of a doughnut type retort. A burner preferably delivers its flame or the heat therefrom into the opening in each coil enclosing means. Each such burner is preferably carried by the support in generally upright position below and generally coaxial with the coil. Desirably burner means separate from the support are also provided, which burner means are arranged to heat the charge and are disposed below the level of the charge.

We further provide, in a furnace, a support for supporting material to be heated, rollers upon which the support is travelable and rail means detachably fastened to the support and adapted to ride on the rollers when the support travels. Extension means preferabl project downwardly from the support, the rail means in such case desirably being detachably fastened to the extension means. The extension means may have enlargement means spaced from the support and the rail means may be clamped to the extension means at the enlargement means.

The furnace preferably comprises, in addition to the support for supporting material to be heated, an I-beam connected with the support at one flange of the I-beam with the other flange spaced from the support, a rail clamped to the second mentioned flange of the I-beam and rollers on which the rail is adapted to ride whereby the support is travelable. The means for clamping the rail to the I-beam flange preferably has a portion extending through the ,railand opposed heads embracing the flange. In the preferred construction a pair of generally parallel spaced apart I-beams is provided, such I-beams being connected with the support with a flange of each I-beam spaced from the support, and a pair of rails is also provided, clamping means clamping the respective rails to the respective I-beams at said flange of each thereof.

We further provide, in a furnace, a support for supporting material to be heated, a plurality of generally parallel series of rollers, a plurality of means including rails interposed between the support and the rollers and adapted respectively to ride on the respective series of rollers whereby the support is travelable and members extending between said means connecting the same together, one only of said means being fastened to the support, the remainder of said means being maintained in place by said members and bearing aromas against the support. In the preferred structure there are two generally parallel series of rollers and the means above referred to are I-beams interposed between the support and the rollers. A rail is preferably connected with each I-beam and adapted to ride on one of the series of rollers. Preferably the rails are detachably clamped to the I-beams.

We still further provide, in a furnace, a traveling support, a charge enclosure on the support, means for heating the charge during traveling of the support, means for introducing gas into the charge enclosure during traveling of the support, a. gas exhaust conduit leading from the charge enclosure and an exhaust gas header disposed along the path of the support, the header having a slot extending generally parallel to the path of the support, the conduit extending into the slot and advancing along the header during traveling of the support. Means are'preferably provided for drawing exhaust gas from the header. A sludge trap may be provided in the conduit for preventing sludge from passing into the header.

We also provide a method of heating a charge in a furnace having a traveling charge support comprising directing stationary heating flames generally transversely of the path of the charge so that said flames pass generally around the charge during traveling of the. support and directing through the charge a heating flame which travels with the support. The charge is preferably a hollow charge and the last mentioned flame is preferably directed into the hollow charge. The stationary flames are preferably directed generally under the support and around the charge. The charge may have a passage therethrough and in such case is preferably arranged with the passage extending generally vertically and the flame which travels with the support is preferably directed through the passage in the charge from the bottom thereof.

The charge may be a cylinder made up of one or more coils arranged with its axis generally vertical. A doughnut type retort may be disposed over they cylinder. Stationary heating flames are desirably directed generally under the support upon which the charge is carried and around the retort during traveling of the support, and the flame which travels with the support is desirably directed generally axially of the retort and so as to extend into the passage therethrough. I

Other details, objects and advantages of the invention will become apparent as the following description of a present preferred embodiment thereof and a present preferred method of practicing the same proceeds.

In the accompanying drawings we have shown a present preferred embodiment of the invention and have illustrated a present preferred method of practicing the same, in which Figure 1 is a partial transverse section through the heating chamber and hearth of a rotating annular hearth annealing furnace; and

Figure 2 is a horizontal sectional view, largely diagrammatic, of the furnace.

Referring now more particularly to the drawings, the apparatus shown comprises a circular hearth II! and a heating chamber I I, C-shaped in plan, concentric therewith, subtending a central angle of somewhat less than 270. The heating chamber is composed of side walls I2 and I3 and a roof I4 built up of refractory brick and insulation and assembled within a suitable binding. The furnace binding includes buckstays I5, posts I la, circumferential beams lib and channels II and I1, and a sheathing of metal plate I8. Burners I! extend into ports 20 formed in the side walls I2 and I3 and spaced circumferentially thereof. The center lines of the several burners are designated IS in Figure 2. The several burners are controlled in any suitable manner to provide a plurality of successive heating and cooling zones hereinafter referred to in greater detail.

The hearth I0 comprises a plurality of segmental base plates 2I supported upon spaced circular beams 22 and 22a which are generally parallel to each other. The beams 22 and 22a are shown as being in the form of I-beams. One only of the beams 22 and 22a is fastened to the base plates 2| The I-beam 22 is fastened to the base plates through the upper flange thereof, the I- beams being arranged, as shown in Figure 1. with their webs extending generally vertical. The I-beam 22 is clamped to the base plates 2I by clamping members 22b through which pass bolts 22c. The bolts 220 also pass through the respective base plates 2|. Thus the circular I-beam 22 is firmly clamped to the segmental base plates 2 I.

The circular I-beam 22a is not fastened to the segmental base plates 2 I. It cooperates with the circular I-beam 22 in supporting the base plates and, as above stated, lies parallel to the I-beam 22 with its web extending substantially vertically. The I-beams 22 and 22a are connected together at intervals therealong by transversely extending connecting members 22d which may be in the form of I-beams or channels and which may have their ends welded to the I-beams 22 and 22a or may be otherwise suitably connected to the I-beams 22 and 22a to maintain the same in substantially fixed relative position. The upper flange of the I-beam 22a lies against the bottoms of the segmental base plates 2I just as the upper flange of the I-beam 22 does, but the I-beam 22 is free to move slightly due to relative expansion and contraction and in so moving may slide relatively to the bottoms of the base plates 2I. Thus a flrm support is provided for the base plates 2I but any danger of buckling due to relative expansion and contraction is avoided.

The I-beams 22 and 22a have rails 23 and 23a removably clamped thereto. Each of the rails 23 and 23a is provided at intervals therealong with bores 23b. Through each such bore extends a bolt 24b. The bolt also passes through opposed clamping members 230, each having a recess 23d for receiving an extremity of the lower flange of the I-beam 22 or 2241 as the case may be. A nut is applied to the bolt and the bolt is tightened to force the clamping members inwardly to flrmly embrace and hold the lower flange of the I-beam. The bores in the rails 23 and 23a are so arranged relatively to the recesses 23d in the clamping members 23c that when the clamping members are drawn up the upper surfaces of the rails lie against and in contact with the bottom surfaces of the I-beams. Thus the rails 23 and 23a are firmly clamped to the bottoms of the I-beams 22 and 22a respectively and to all intents and purposes coact unitarily therewith but are removable and replaceable by rendering inoperative the clamping mechanism above described.

The rails 23 and 23a travel on spaced pairs of supporting and driving rolls or rollers 26, one

. of which is flanged, journaled in bearings 21 are driven by radial shafts 29 extending inwardly to a centrally located drive mechanism (not shown). The flanged rolls 28 form in effect a circular series of rolls or rollers'underlying and supporting the rail 23 and the unflanged rolls 26 form in effect a circular series of rolls or rollers underlying and supporting the rail 2321, the two series of rolls being substantially parallel. On rotation of the shafts 29 the hearth I is caused to travel progressively through the heating chamber ll of the furnace. The base plates 2| have sealing channels 3| formed on the edges thereof and sealing flanges 32 extend downwardly from the lower edges of the side walls l2 and I3 into those channels. The heating chamber is thus effectively sealed at all points between its two open ends.

The hearth It includes a refractory concrete base 33 and a layer 33' of refractory brick on which piers or stools 34 are laid up in spaced relation circumferentially of the hearth. The piers or stools are preferably metal tubes of oval shape in section. A charge support in the form of a base 35 is disposed on each set of stools 34 and is adapted to receive a charge of material to be heated, e. g., a coil 36 of metal strip to be annealed. A cover 31 is disposed over the coil or coils on each base 35, thereby providing a complete enclosure and retort therefor. The retort is of the so-called doughnut type having a central passage 31a which is substantially coaxial with the coil. The cover 31 has an outer annular wall 31b and inner annular wall 310 coaxial therewith and a top annular wall 31d joining the upper ends of the walls 311) and 310. The central passage 31a is formed by the inner wall 310.

Outer and inner sealing channels 38 and 38a are formed in the base 35 which has a central opening 38b therethrough. The respective sealing channels 38 and 38a contain sealing material 380, as, for example, sand. The bottoms of the respective Walls 31b and 310 'of the cover 31 are disposed in the channels 38 and 38a and are sealed therein by the sealing material 33c.

Radial baflie walls 38' are disposed on the hearth in circumferentially spaced relation to limit outflow of the heated atmosphere from the interior of the furnace and maintain the temperature therein more nearly uniform. It will be noted that the bases 35 are disposed at a level above that of the burners Hi. The purpose and effect of this relative disposition will be referred to later.

Mounted in the hearth at intervals thereabout, one below and coaxial with each base 35, are burners 38d. Each burner is arranged substantially vertically and has its lower or fuel inlet end let out through the base plate 2| where it is provided with a connecting device 28a for a flexible hose. Disposed above the shafts '29 and extending circumferentially of the furnace throughout a portion of the circumference thereof is a fixedly mounted pipe 38 for supplying fuel for the burners 38d. At intervals the pipe 38/ is provided with valved offtakes 38g, each having a connecting device 38h for a flexible hose. There may be as many valved offtakes 389 as there are burners 38d or there may. be fewer valved oiftakes; in the latter case the flexible hose connected with each valved offtake may 1 have branches leading respectively to more than one burner. Fexible hose 381' is shown in the drawings.

Since the hearth rotates while the pipe 38! does not rotate it will be apparent that no permanently fixed connections between the pipe 33! and the burners 38d can be employed. Due, however, to the very slow movement of the hearth, it is feasible to employ flexible hose con nections between the pipe 381' and the burners, which connections may be connected with successive burners as the hearth rotates. For example, one hose may be connected to a certain burner while that burner is approaching, passing and leaving one valved offtake, the annular travel of the hearth through which the connection may remain effective being determined by the length of the hose. When a hose is to be disconnected from a burner the valve controlling the valved oiftake to which that hose is connected is closed, and when the hose is connected to another burner the valve is again opened. The amount of fuel admitted to the burner may be controlled by regulating the valve at the valved offtake from the pipe 33f. Air may be admitted to the burner in the usual manner. Thus there are provided in addition to the stationary transverse burners I! the vertical burners 38d carried by the hearth and one mounted below and coaxial with each base 35.

It is desirable to maintain a protective atmosphere surrounding material being annealed to prevent atmospheric oxidation thereof at elevated temperatures. We accordingly provide means for supplying such atmosphere to each coil enclosure or retort continuously throughout its travel through the heating chamber H. We provide an annular channel 39 extending around the furnace under the hearth l0 having spaced flanges 40 and adapted to be partly filled with sealing liquid as shown in Figure 1, thus forming continuous gas-supply chambers 4| and 42. Pipe connections 43 and 44 extend from these chambers to any convenient source of the gas to be used'as the atmosphere in the coil enclosures.

In order to convey the desired gas atmosphere to the individual coil enclosures, we provide pipe connections 45 for each enclosure extending vertically through the hearth and the base 35. A valve 46 is inserted in each piping connection and branches 41 extending downwardly therefrom communicate respectively with the chambers 4| and 42. Thus by shifting the valve 46 one or the other of two special-atmosphere gases may be supplied to the interior of the coil enclosures. Since the channel 39 and the chambers 4| and 42 are continuous the coil enclosures are maintained in communication with the gas supply throughout their travel around the full circle of the hearth.

Vent pipes 48 extend upwardly through the hearth and the bases 35, one of such pipes being provided to vent each coil enclosure. Each'of the pipes 48 leads to a T 48a to which is connected a downwardly extending sludge trap 48!) having a removable closure 48c. Extending horizontally from the T 48a is a pip 48] which leads to a valve 49. Leading from the valve 49 is an exhaust pipe 49a which enters an exhaust gas header 492) through a slot 490 therein, which slot extends generally parallel to the path of the hearth. One or more pipes 49d is or are connected with the exhaust gas header 4%. The pipe or pipes 49d may lead to a suctio pump or pumps for drawing the exhaust gas from the header 49b. Since the slot 490 is parallel to the circular path of the hearth and since the pipes 49a travel with the hearth, the pipes 49a always enter the slot 49c so that in all relative rotative positions of the pipes 49a and the header 4% as passing downwardly through the pipes 48 enters the header "b. The amount of gas passin; out may be controlled by regulating the valves 4!. Any sludge passing down through the pipes ll collects in the respective sludge traps 48b and may be removed by removing the closures "e. The pipes #8 and the exhaust mechanism just described permit the coil enclosures to be purged of atmospheric air as the special-atmosphere gas is admitted.

Referring to Figure 2, if the hearth It be assumed to rotate in the direction indicated by the arrow 50, the portion of the travel of the hearth adjacent the entering end of the heating chamber may be considered the charging zone. While a given charge support is in this zone the coil or coils constituting the charge to be annealed may be placed thereon and a cover 31 disposed thereon. The valve 45 of the retort or enclosure in question is then opened to admit the special atmosphere, after opening th valve 4! to permit the escape of atmospheric air trapped in the enclosure. The hearth is driven at a speed such that it makes a complete revolution in a matter of forty-eight hours or so. There is thus ample time to attend to the necessary preliminaries after depositing and enclosing the charge and before the sealed retort reaches the entering end ii of the heating chamber. After the retort or enclosure has been thoroughly purged of atmospheric air the valve 49 is closed. The valve 4', however, is left open to maintain the retort in communication withthe source of protective atmosphere. We preferably limit the height of the charg when disposed on its supporting base to a value less than its maximum diameter and preferably to a value about sixty per cent. and not more than seventy-five per cent. of the maximum diameter. This limits the thicknes oi metal through which heat must fiow in the direction in which the mass is most easily penetrable, i. e., parallel to the axis of the coil. The charge, of course, may be composed of one or more coils, but in either case the ratio between the height of the charge and its maximum diameter as given above should be maintained for best results.

The burners I! nearest the entering end of the heating chamber are fired in such a manner as to maintain a preheating zone extending over the first few feet of the length of the chamber. At the same time the burners 38d beneath the respective enclosures are fired as they enter the end if of the heating chamber. The charge is subjected to rapid heating in this zone in order to cause its temperature to start quickly upward. The rate of heating the charge in the preheating zone is preferably as high as possible with due regard for the danger of damaging the retorts by burning or the like.

From the preheating zone the retort passes into and through a rapid heating and soaking zone extending over about half the circumferential length of the heating chamber. In this zone the burners I! and "it are adjusted to fire so as to cause the temperature of the charge to rise as rapidly as possible to the desired maximum, after which sufiicient time is permitted for the temperature of the charge to equalize throughout the mass thereof without further increase. The rate of heating attainable in both the preheating zone and the rapid heating zone is high because the combustion gases from the burners l9 and "d have access to the retort at the bottom, the sides, the top and the central passage thereof. This fact, together with the limitation on the 8 height of the coil or coils, causes the heat to flow into the central parts of the coil very rapidly from the edges of the successive turns thereof. The rate of heat inflow in this direction is much greater than that which takes place radially or normal to the successive turns of the coil.

After the charge has been brought to the desired maximum temperature and the temperatures of all portions thereof have been equalized. th heating stage of the annealing cycle has been completed and the charge is ready for gradual cooling. To this end we maintain in the remainder of the circumferential length of the heating chamber II a slow cooling zone and a rapid cooling zone. In the slow cooling zone the burners l8 and 38d are adjusted to supply a very small amount of heat, i. e., less than that necessary to maintain the temperature of the charge against decrease by natural radiation. As a result the temperatur of the charge decreases at a relatively low rate while it traverses the slow cooling zone. We effect a more rapid cooling of the charge while it traverses the final or rapid cooling zone by blowing air into the exit end 52 of the heating chamber as indicated by arrows 53. The air blast delivered through the exit end serves not only to cool th charge traversing the rapid cooling zone but also to cool the charge while passing through the slow cooling zone. A suction hood located adjacent the entry end of the furnace chamber exhausts the spent combustion gases therefrom and delivers them to a stack.

By the time the charge reaches the exit end 52 of the furnace chamber it has been cooled below the temperature at which atmospheric oxidation proceeds rapidly. The valve 46 may therefore be moved to closed position and the cover 31 removed. The charge is then lifted ofi by appropriate means while its support is traveling through the unloading zone. When the charge has been removed the supporting base is ready to receive another coil of material to be annealed and the entire cycle is then repeated.

It will be understood that the travel of the hearth may be continuous as described above so that fully annealed charges are being delivered from the exit end of the furnace successively while coils of strip to be annealed are being disposed successively on the charge supports as they traverse the charging zone. In a particular operation the charges may be heated to a temperature of about 1350 F. in the rapid heating and soaking zone, cooled to about 1250 F. in the slow cooling zone and finally cooled to about 500 F. or below in the rapid cooling zone. It will be understood, however, that the temperature limits in various portions of the hearth travel may be varied widely, depending upon the characteristics 60 desired in the fully annealed product.

traveling may be greater than when the hearth is moved continuously. In intermittent operation the hearth is preferably stopped when the charge supports occupy positions relative to the burners I! such that the burners fire between the piers or stools 34 and not directly against them, there'- by avoiding excessively rapid deterioration of the stools under the high temperatures at which they are subjected. Also in such positions the burners l8 and 38d combine to form substantially an envelope of flame and hot gases about the retort whereby the charge is heated at high efllciency.

The burners i9 and 38d combine to heat the charge or charges on the rotating hearth with unprecedented economy and uniformity. The flames from the burners is are directed generally under the charge enclosures and around the same during traveling of the hearth, and the flames from the burners 38d are directed into the passages 31a. Thus the charges are heated uniformly and with remarkable economy.

While we have shown and described a present preferred embodiment of the invention and have illustrated a present preferred method of practicing the same, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

We claim:

1. In a furnace, a travelling hearth adapted to support a hollow coil to be heated, an annular enclosure around and coaxial with the coil, said enclosure having an axial passage therethrough, and a burner carried by the hearth directed to ward said axial passage in order to heat the inner portion of the coil.

2. In a furnace, a travelling hearth, supporting means on the hearth, a generally annular enclosure having a space extending therethrough along its central axis and adapted to contain a coil to be heated, said enclosure being spaced above the hearth and resting on the supporting means, and a burner carried by the hearth below the enclosure and directed toward the space extending through the enclosure in order to heat the coil.

3. In a furnace, a travelling hearth, supporting means on the hearth, a generally annular enclosure having a space extending therethrough along its central axis and adapted to contain a coil to be heated, said enclosure being spaced above the hearth and resting on the supporting means with the central axis of the enclosure vertical, and a burner carried by the hearth below the enclosure and directed vertically toward the space extending through the enclosure in order to heat the coil.

4. In a furnace, a travelling suppl't adapted to support a charge, burner means separate from the support arranged to heat the charge, said burner means being disposed below the level of the charge, and burner means carried by the support also disposed below the level of the charge and disposed to heat portions of the charge not directly heated by the first mentioned burner means.

5. In a furnace having a travellinghearth enclosed by a stationary hood, the improvement comprising supporting means on the hearth to carry a hollow charge spaced above the hearth, an annular enclosure around and coaxial with the charge, said enclosure having an axial passage therethrough, a burner positioned in the hood at a level above the hearth and below the charge and directed toward the space between the hearth and the charge, and a burner carried by the hearth below the charge and directed toward the axial passage of said annular enclosure.

6. In a furnace, a rotary hearth disposed and rotatable generally in a horizontal plane for supporting at intervals therealong coils to be heated with the axes of the coils generally vertical,

means enclosing each coil but having an opening therethrough generally along the axis of the coil, burners carried by the hearth in generally upright position below and generally coaxial with the coils and adapted to heat the insides of the coils and other burners separate from the hearth disposed with their axes generally horizontal and adapted to heat the outsides of the coils.

7. In a furnace, a travelling hearth disposed and movable generally in a horizontal plane for supporting at intervals therealong coils to be heated with the axes of the coils generally vertical, burners carried by the hearth in generally upright position below and generally coaxial with the coils and adapted to heat the insides of the coils and other burners separate from the hearth disposed below the coils with their axes generally horizontal and adapted to heat the outsides of the coils.

8. In a furnace, a support for supporting material to be heated, a plurality of generally parallel series of rollers, a plurality of means including rails interposed between the support and the rollers and adapted respectively to ride on the respective series of rollers whereby the support is travelable and members extending between said means connecting the same together, one only of said means being fastened to the support, the remainder of said means being maintained in place by said members and bearing against the support.

9. In a furnace, a support for supporting material to be heated, two generally parallel series of rollers, two I-beams interposed between the support and the rollers, a rail connected with each I-beam and adapted to ride on one of the series of rollers whereby the support is travelable and members extending between the I-beams connecting the I-beams together, one only of the I-beams being fastened to the support, the other I-beam being maintained in place by said members and bearing against the support.

10. In a furnace, a support forsupporting material to be heated, a plurality of generally parallel series of rollers, a plurality of thrust members interposed between the support and the rollers, a rail detachably clamped to each thrust memher and adapted to ride on one of the series of rollers whereby the support is travelable and means extending between the thrust members connecting the thrust members together, one only of the thrust members being fastened to the support, the other thrust member being main tained in place by said means and bearing against the support.

11. A method of heating a charge in a furnace having a traveling charge support comprising directing stationary heating flames generally transversely of the path of the charge so that said flames pass generally around the charge during traveling of the support and directing through the charge a heating flame which travels with the support.

12. A method of heating a hollow charge in a furnace having a traveling charge support comprising directing stationary heating flames generally under the support and around the charge during traveling of the support and directing into the hollow charge a heating flame which travels with the support.

13. A method of heating in a furnace having a traveling charge support a charge having a generally vertical passage therethrough comprising directing stationary heating flames generally under the support and around the charge during traveling of the support and directing through 11 said passage from the bottom thereof a heatin flame which travels with the support.

14. A method of heating in a furnace having a traveling charge support a hollow cylinder made up of one or more coils arranged with its 5 axis generally vertical comprising disposing a doughnut type retort over the cylinder, said retort having an axial passage therethrough, directing stationary heating flames generally under the support and around the retort during traveling of the support and directing generally axially of the retort and so as to extend into the passage therethrough a heating flame which travels with the support.

DAVID P. COOPER. EARNEST D. SEVENICH.

12 REFERENCES orrEn The following references are of record in the tile of this patent:

UNITED STATES PATENTS Number Name Date 1,903,909 Cope et al Apr. 18, 1933 2,074,662 McLay Mar. 23, 1937 2,417,063 Cooper Mar. 11, 1947 FOREIGN PATENTS Number Country Date 323,097 Germany July 15, 1920 

